Use of ferroelectric material for the tuning element is one way of achieving electrically tunable microwave devices such as varactors, resonators, filters and phase shifters, whose operating characteristic can be altered by applying an electric field. Ferroelectric thin film based tunable microwave devices have the advantages of high switching speed, compactness, light weight, low power consumption, and high reliability relative to competing technologies.
Tunable planar ferroelectric devices are usually implemented on a substrate whose entire surface is covered by a ferroelectric thin film. High temperature super-conductors are often used for the conductive layers to obtain better performances such as very low insertion loss. Ideally the ferroelectric thin film should be patterned so that it is present only on selected areas of a substrate 11 (FIG. 2), rather than on its entire surface (FIG. 1) so as to reduce insertion loss due to microwave dissipation in the ferroelectric thin film as well as unintentional tuning from regions of the ferroelectric thin film where tuning is not wanted.
In FIG. 1, ferroelectric layer 12 is seen to comprise two regions—12b where tuning of the device is intended to take place and 12a which extends outside of and away from 12. Unwanted tuning can occur in 12a where it is close to electrodes 13.
One approach to achieving ferroelectric thin films on only selected regions has been to use “drop-in” pieces of ferroelectric thin film that were grown on a separate substrate and subsequently attached to another substrate with the patterned conductive layer circuit. However this approach usually requires an unacceptably large fabrication error tolerance and greatly complicates the fabrication process.
Another approach that has been used in the prior art to achieve a patterned ferroelectric thin film on a substrate has been to use chemical etching or ion beam milling. However, patterning a ferroelectric layer by chemical etching presents problems as ferroelectric thin film materials are very inert and chemicals capable of etching them will also etch the substrate or/and the conductive layers.
Thus, conventional chemical etching processes or ion beam milling will lead to a damaged substrate surface and/or conductive layer, resulting in a fabricated device that has poor performance. This problem of damaging the substrate surface and/or the conductive layer is especially severe in cases where a high temperature super-conductor is used for the conducting layer(s).
Conventional lift-off processes for patterning the ferroelectric thin film using an organic photoresist masking layer are also unsuitable due to the high temperature required for the deposition of the ferroelectric thin film.
A routine search of the prior art was performed with the following references of interest being found:    1. M. J. Lancaster, J. Powell, and A. Porch, “Thin-film ferroelectric microwave devices”, Supercond. Sci. Technol., 11 (1998), 1323-1334.    2. R. A. Chakalov et al, “Fabrication and investigation of Yba2Cu3O7-δ/Ba0.05Sr0.95TiO3 thin film structures for voltage tunable devices”, Physica C, 308 (1998), 279-288.    3. F. A. Miranda et al, “Design and Development of Ferroelectric Tunable Microwave Components for Ku- and K-Band Satellite Communication Systems”, IEEE Trans. Microwave Theory Tech., 48 (2000), 1181-1189.